Rollup merge of #63833 - estebank:suggest-closure-call, r=petrochenkov

Suggest calling closure with resolved return type when appropriate

Follow up to #63337. CC #63100.

```
error[E0308]: mismatched types
  --> $DIR/fn-or-tuple-struct-without-args.rs:46:20
   |
LL |     let closure = || 42;
   |                   -- closure defined here
LL |     let _: usize = closure;
   |                    ^^^^^^^
   |                    |
   |                    expected usize, found closure
   |                    help: use parentheses to call this closure: `closure()`
   |
   = note: expected type `usize`
              found type `[closure@$DIR/fn-or-tuple-struct-without-args.rs:45:19: 45:24]`
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index da66fdf5b1b..f41fffe507d 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -385,7 +385,7 @@ impl<'tcx> ClosureSubsts<'tcx> {
         let ty = self.closure_sig_ty(def_id, tcx);
         match ty.sty {
             ty::FnPtr(sig) => sig,
-            _ => bug!("closure_sig_ty is not a fn-ptr: {:?}", ty),
+            _ => bug!("closure_sig_ty is not a fn-ptr: {:?}", ty.sty),
         }
     }
 }